Noise reduction device in radio receivers



July 29, 1947-.

l. W. CONRAD NOISE REDUCTION DEVICE IN RADIO REcEWE zs File d Sept. 22, 1945 5 Sheets-Sheet l 58 LOCAL OSC/LLATOR CONTROL NETWORK IN VEN TOR.

y 1947- 1. w. CONRAD NOISE REDUCTION DEVICE IN RADIO RECEIVERS Filed Sept. 22, 1943 3 Sheets-Sheet 2 I 52 90 PHASE RELATIVE 27,20 ORIGIN OF Tms AXIS 3M 21/. CW1

IN V EN TOR.

July 29, 1947. 1. w. CONRAD NOISE REDUCTION DEVICE IN RADIO RECEIVERS Y 3 Sheets-Sheet 5 Filed Sept. 22, I945 ORIGIN 0F TIME AXIS \i/Ww INVENTOR.

Patented July 29, 1947 UNlTED STATES FATE NOISE REDUCTION DEVICE IN RADIO RECEIVERS 21 Claims.

The present invention relates to radio reception and has for its principal object to provide relatively simple, practical means for reducing interfering electrical noise of either natural or man made origin, normally encountered in the reception of radio telegraph and radio telephone signals. Another object is to accomplish this result with equipment which can readily be added to existing radio equipment.

Essentially the invention contemplates the use for mixer or detector functions in a radio receiver, of a balanced. modulator circuit in which essentially all amplitude modulation on the incoming radio signal is balanced out, except insofar as this cancellation may be made intentionally incomplete to permit desired modulation to be received. When adjusted for complete cancellation, no output signal voltage is derived from the circuit from either an incoming signal or from undesired electrical noise; if, however, the cancellation be made only partially complete, essentially all amplitude modulation components above an arbitrary, variable level may be made to balance out, while those components below this same level are passed to the output. In this manner the received signal may be freed of noise and interference components above the selected signal or modulation level.

With the foregoing general object in View the invention consists in the novel combinations and arrangement-s of features as will be hereinafter more fully described, illustrated in the accompanying drawings and defined in the appended claims.

In the accompanying drawings, wherein are illustrated different practical embodiments of the invention and wherein like characters of reference denote corresponding parts in related views:

Figure 1 is a diagrammatic view illustrating one practical embodiment of the invention utilizing a balanced diode rectifier.

Figures 2 and 2B are diagrammaticviews illustrating the action of the circuit of Fig. 1 upon an incident mixture of noise and desired signal, when adjusted for the reception of CW or radio telegraph signals.

Figures 3 and 3B are diagrammatic views illustrating the action of the circuit of Fig. 1 upon an incident mixture of noise and a desired modulated signal, when adjusted for the reception of amplitude modulated radio telephone signals.

Figure 4 reflects diagrammatically another practical embodiment of the invention wherein the balancing voltages are derived from the same envelope of the incoming signal and noise, resulttransformer ing in independence of incoming signal symmetry for the desired cancellation.

Figure 5 is a diagrammatic view illustrating another practical embodiment of the invention utilizing triode modulator elements.

Figure 6 is a diagrammatic view reflecting still another practical embodiment wherein multi-grid modulator elements are used.

' connected through radio frequency chokes ID and H respectively to equal load impedances l2 and 13. In addition, cathodes 8 and 9 are connected respectively to the junction IQ of loads l2 and [3 by equal radio frequency by-pass condensers it and I5. A second radio frequency transformer is shown at It, the secondary I! of which is connected between center tap l8 of input transformer secondary It and junction'lt. The primary 2!! of It is excited by radio frequency source 2i, and the primary 2 of transformer l is excited by signal source 22. Source 22 ordinarily'will be the radio frequency or intermediate frequency sections of a conventional radio receiver, but may be some other source, as for example, an antenna. Source 2| ordinarily will be a self excited local oscillator and associated phase shifting network, but may be connected as shown to source 22 through switch S and a hetercdyne oscillator-limiter amplifier 38, whereby the frequency of 2! may be controlled by source 22 in a manner well known to the art, either to lock in on the identical frequency of 22, or to become operative on a slightly different frequency from 22 only when 22 is operative. With respect to center tap I8, the diode plates 8 and 1 will be excited out of phase by signal source 22, whereas cathodes 8 and 9 will be excited in phase with each other by source 2!. Relative excitation of plates 6 and I may be varied by varying the coupling between primary 2 and the corresponding halves of center tapped secondary t to secure the desired degree of balance. Excitation of cathodes 8 and 9 may be varied by varying the coupling between primary 2%) and secondary ll. Output voltage is taken between terminals 23 and 25 connected to loads l2 and I3 respectively through equal coupling condenser-s 25 and 25. At

T is shown a single-pole double-throw switch which, when thrown to the other position, removes the excitation by source 2| from one of the cathodes 8.

The action of the circuit of Fig. 1 is best visualized by plotting voltages and currents at various critical points, and in Figs. 2 and 2B this has been done diagrammatically for the conditions under which the circuit of Fig.1 1 is suited for the detection and reception of radio telegraph signals. Referring to Figs. 2 and. 2B, which are related to Fig. 1, there is represented by line 21' the signal voltage from source 22 on diode gplate -6 (Fig. l) and by solid line 28 the signal voltagepn diode plate 7 (Fig. 1). In general theselines represent a uniform sine wave signal; however, at 29 and 36 are shown the efiiects of a modulation peak of interference or static. The envelope of the signal from 22 is shown as X in Fig. 23. Line 3'! represents the voltage on cathodes 8 and Q (Fig. 1) resulting from sourceii,.operating. on a frequency slightly different Lfroni the signal frequency of 22. Lines '3Lrepresent the. resulting pulses of current through cathode .8, and the effective envelope ofthese pulses appears as a potential at terminal23 by virtue of the-action of load l2. This envelope'is showninFigs. Zand 2B diagrammatically as line332. .Similarly, lines 33 represent the resultant pulses .of current through cathode 8 and the elfectiveenvelope .34 of these pulses appears as apotential at terminal 24. Since these-envelopes are 180 out ofphase, itis thus apparent that an alternating electronictive-force appears between points 23 and .24, the frequency of which is the familiar beat or difference frequency between the frequencies of sources 22 and 2|. However, itisequally apparcut that as a result of signal peaks 2!! :and 30 respectively, at 35 and '36 peaksoccur, the envelopes of which are in phase with each other, and which therefore, tendto cancel when measured'between points23 and 2'4. Cancellation will be'most nearly complete during the periods when a 90 phase difference exists-between excitation from sources 2! and :22 asmay'beseen fby reference to Fig. 2B, where it willbe noted that envelopes-32 and 34 crosseach other (i. e., have equal values and therefore cancel completely) at those points where such a 90 phase diiference exists between cathode excitation 31 and diode'plate excitation curves -21, 28. This is further shown in the mathematical discussion hereinafter set forth,

The net efiect'of the circuit shown in Fig. 1, then, under the conditions imposed, is to produce a heterodyne frequencybetween terminalszii and 24, while effectiyelycancelling all incoming'signal modulation effects from source22 that exceed an arbitrary level. That this arbitrary level is determined by the amplitude of-the-excitation from source 2!, may be seen by letting the amplitude of 3'! in Figs. 2 and 2B diminish, approaching zero. The amplitudes of envelopes 32 a'nd'34 vary accordingly, approaching each other as adjacent pulses 2| and 33 become more and more nearly equal and as adjacent peaks 35 and 3% become more and more nearly identical in amplitude (height), with essentially complete cancellation occurring. Under the conditions describedtherefore, the circuit of Fig. l is suitable for the reception of radiotelegraph signals, with a high degree of noise rejection. 7

A still higher degree of readability of radiotelegraph signals is obtained by deriving source 2| from the radio telegraph signal source 22 through and 'the' resultant noisein these intervals is less than when the cathodes 8 and 9 are excited continuously. The result is a more clear cut distinction between marking and spacing intervals, and

.is-correspondingly easier to read.

,From examination of Figs. 2 and 218, it will be further apparent that the excitation from source 2| may be applied to cathode 8 or cathode 9 singly, rather than to both, with essentially the same result, with the exception that only one envelope 32 or i l will be effective in producing the desired beat note. The other envelope dezgoneratesinto a steady potential, except as it is .afiected by modulation peaks on the incoming signal-fromeource 22, i. e., it becomes the envelope of ,theoriginal signal from source 22 as shown at .Z in ;Fig. 2B. Cancellation of excessive modulation effects, therefore, occurs as before.

For exomplaifswitch'Tin Fig. 1 be thrown to the other position, excitation bysource2l will be removed from cathode 8 and thus only'cathode 9 will be excited. lnjthis event, diode'li, 8 acts as an ordinary detector, and :the potential across load resistor l2 will then follow the envelope X of the original signal from 22, as shown at Z; i. e., envelope 32 becomes envelope Z. Sinceexcitation is 'still on cathode 9, envelope 31% remains unchanged. Thus the potential difference between points 23 and 24 :now reflects the difierence between envelope Z and envelope 34, which, as

shown in Fig;;2B,still reproduces the beat note, and still substantially cancels'modulation peaks of noise :and interference.

:In Figs. 3 and 3B which are also rclatedito Fig. .,:there is represented diagnammatically'the action of the circuit shown in Fig. 1, upon an incoming amplitude modulated radio signal from source 22 when the frequency of source -21 is locked ingsynchronism with. source-22 andjs either 0 or 180 out of phase with 22. The line 39 reflects the modulated signal potential on diode plate 5 (Fig.1) the solid line 4!! reflects the modulated signal potential on diode plate-i (Fig. l) and the line '4! refiectsthe potential on cathodes 8 and 9 (Fig. 1) produced by source 2|. At 42 and 43 are shown the potentials produced on plates 6 and '1 respectively by an interference noise-pulse. The envelope of the original signal from 22 is represented by line'X in Fig. 313. At 44 and 45 are shown the'resulting pulses of cathode current from cathodes 8 and 9 respectively. The effective envelopes of these pulsesshown as 46 and 47 [appear at terminals 23 and 2 1 (Fig.1.), respectively, as potentials through the action of loads 12 and 13. From inspection of the envelopes 46 and 41, it is evidentthat they follow the original signal 'modulation envelope X and are 180 out of phase with each other, so long asthe signal modulation peaks do not exceed in amplitude the amplitude of cathode potential ii. However, it is further apparent asshown by peaks A8 and 4,9, that when the signal-modulation peaks :with eachother and thereforetend to cancel each other when measured .as a potential difference between terminal-s 23 and 24. Therefore, the effect of the circuit shown in Fig. 1 upon an amplitude modulated signal from source 22 under the conditions stated is to produce between output terminals 23 and 24 a potential difference which accurately follows the original signal modulation envelope up to the point where the modulation peaks equal in amplitude the cathode excitation potential derived from source 2 l; and to effectively cancel all modulation above that point. Phase reversal of envelope 46 may [be seen to occur at point Y where the amplitude of the signal X from 22 becomes equal to that, 4|, from source 2 I From the foregoing discussion and reference to Figs. 1 thru 3, it will be apparent to those skilled in the art that the excitation from source 2| may be applied to cathode 8 alone rather than to both 8 and 9, as for example by a'switch arrangement similar to that shown at T in Fig. 1, but connected to cathode 9 instead of 8, with essentially the same result under the phase relations shown. In this case, potential 4| vanishes for cathode 9; i. e., diode l, 9 then acts as an ordinary detector, the output of which follows the impressed modulation envelope as shown at Z in Fig. 3B, the phase relation of which in regard to envelope 46 is identical to that of 41. However, it is not possible to excite cathode 9 only under the phase relation shown, becauseto do so would result in the inversion of envelope 46, that is, envelope. 46 would then become envelope Z, so that both envelopes 46 and 41 would be completely in phase and would cancel each other for all modulation values.

In the embodiment reflected by Figs. 1, 2, 2B, 3, and 3B, rectification or detection of opposite half cycles of the incoming signal has been utilized to furnish opposing potentials which are so added as to cancel modulation effects above an arbitrary, variable value. Obviously, the completeness of cancellation is partially dependent on the symmetry of such opposite half cycles for equivalent opposing potentials, and any non symmetrical excursions of the incoming signal will a result in an uncancelled potential between output terminals 23 and 24.

This dependence upon the symmetry of the incoming signal is avoided in the embodiment shown in Fig. 4 where the input signal from source 22 is impressed on diode plates 50 and 5| in the same phase thru the agency of a radio frequency transformer 52, and associated tuning condenser 53. Cathodes 54 and 55 are excited 180 out of phase from source 2| through the agency of transformer 56 and coupling condensers 51 and 58. Cathodes 54 and 55 are also coupled through radio frequency chokes 59 and 60 to equal radio frequency bypass condensers 6| and 62 respectively and to essentially equal load impedances 63 and 64 respectively. Output is taken from terminals 65 and 66 through coupling condensers 61 and 68.

With the circuit of Fig. 4, therefore, since both plates 50 and 5| are driven positive by identical half cycles of the signal from source 22, exactly the same modulation and noise components will affect each, and under conditions of no excitation from source 2 I, complete cancellation of all modulation effects may be obtained by balancing variable adjustment 69 to produce equal voltage drops across loads 63 and 64, If, now, cathodes 54 and 55 are excited by source 2| at a frequency slightly different from that. of signal source 22, the modulation products or beat frequencies will appear as potential differences between 65 and 66, with complete cancellation of all signal modulation peaks above an arbitrary level, determined by the amplitude of the excitation from source 2|, in a manner very similar to that described above for the operation of the circuit of Fig. 1. Under the conditions indicated, therefore, thecircuit of Fig. 4 is suited for the reception of radio telegraph signals, and as in the case of the circuit of Fig. 1, the effectiveness may be increased to some extent by making source 2| operative only when actuated by source 22 through the agency of a heterodyne oscillator 88, as described under the operation of the circuit of Fig. 1.

Likewise, in a manner analogous to that explained for the circuit of Fig. 1 and graphically represented in Figs. 3 and 3B, the circuit of Fig. 4 may also be adapted to the reception of radio telephone signals by synchronizing local source 2| with signal source 22, and applying the excitation from source 2| to cathodes 54 and '55 in either 0 or 180 phase difference with the excitation on plates 50 and 5| from signal source 22. Under these conditions th amplitude modulation envelope of the signal from source 22 will be recovered as a potential difference between terminals 65 and 66, up to the point where the modulation peaks exceed the amplitude of the excitation of cathodes 54 and 55 from source 2|. Above this point, the amplitude modulation envelope of the signal from source 22 will be essentially balanced out.

Again, as in the case of the circuit of Fig. 1, the desired effect may also :be obtained by applying the excitation from source 2| to a single cathode rather than to both 54 and 5-5 as for example by opening switch V in Fig. 4. For radio telephone operation, the excited cathode must be in phase with its corresponding diode plate.

Application of the principle of the invention is notlimited to diode detection or rectification, and, accordingly, in Fig. 5 is illustrated diagrammatically a practical embodiment of the invention wherein the excitation from signal source 22' is applied through'a radio frequency transformer 13 to the control grids l9 and ll of a balanced dual triode circuit. With respect to the center tap 15 of transformer secondary 14, grids l0 and H are driven 180 out of phase by source 22, and cathodes l6 and H are driven in phase with each other by local frequency source 2| through the agency of transformer 18. Variable condenser 12 serves to tune secondary 14 to the desired signal frequency of source 22. Grids l0 and H normally are biased essentially to out off by potential source l9.' Plates and 8| are by-pas'sed to cathodes l6 and l! by equal condensers 82 and 83 respectively, and are coupled to essentially equal load impedances 84 and 85 through radio frequency chokes 86 and 8? respec. tively. Output is derived as potential difference between terminals 88 and 88 through coupling condensers 90 and 9|. Plates 80 and 8| are made positive with respect to cathodes l6 and 11 by potential source 92. Under conditions of no excitation from source 2|, and equal excitation on grids 10 and H from source 22, it will be apparcut that no symmetrical modulation products will appear between points 88 and 89. Plate current will flow through both loads 84 and 85, but under the conditions stated, they will be opposed in voltage effect and thereby cancel between points 88 and 89. Upon adding excitation from source 2|, when 2| is slightly different in frequency from that of source 22, modulation productsrbetwecnz; t hBl frequencies: o source 1215 and:

source ZZsW-ill appearzbetween'the output termi-.

the amplitude of; the excitation: from source 2 l again-twill remaincancelled, since they -neces-, sarilyawill. be in phase :withreacn other; and are added. 'nzsoppositlonlt Under; the conditions vdescribedethereiore the ;circuit vof Fig. 5, issuita-ble for the receptionaof radio telegraph signals, with a; high.:;degr ee oi:- incoming; noise rejection; and as.;belior e, .the effectivenessof the signal ,reception may be improved by making source 21 operative only; when? actuatedrby, source 22 through the? agency; of; a heterodyne;.aoscillator :38.

From .the above discussion of the E principle :iofthednventiom: it: is; believedthat it :will now be appal enttoithose skilled in the; art that-by synchronizingzthe frequency of localssource=2l with signal sourceZZaand-xzby applyingthe excitation from.soume 2l?tor;cathodesz'lfijrandill;: 180? out: of phasetwithi the signal 'sourceq22 excitation ;-.of; rids; 1.0.1 013111; all.gamplitudeimodulation on, the signal from-szl whichi lies below an arbitrary level determinedrzby: the=-amplitude :of the 1 excitationfrom-source :2 iswillpbesrecovered. between tenninals-z88giand 89;; and: that; essentially all ,signal modulation .ahoveithisrsame arbitrary levelwill be. balanced." out time. manner analogous, to that. explained fort-the circuit of; Figtzlt and graphically represented in Figs. 3 and 3B. Under these conditi-ons; theocircuit:= of, Fig; 5 is; suitable for the reception of; radio.:telephone signals.

Alternatively; it: is. to: .beanoted that grids I0 and; I l" rotzFigi; 5,; rather than being excited-1 180 outsofs'phaseewithmeaoh: other by source- 22 (as ShOWILiIIlflJ bevexcitedrin phas.e,;and that upon exciting cathodeaflfi sand 71: 180. outof phase Withgeach other. fromvtsource- 2|; the action resultingawill-gbei essentially the 1 same. as; for. the circuit:shownrinzl fi'g.-- 5,? except; that thabalancingr' potentials across I the output terminals 1 are their; derived from identica1:;half cycles of the signah from-source- 22: and:. therefore no not de-. pend :upon the symmetry 'of the amodulation eX: istingtontthis-signal, from Zlfonrequivalence 'and perfection'of cancellation;

Sincerthe :principleof. theinvention; involved in;;the icircuit'iofxFige .5 fisminherently similar to thaizofrrFigs-j ,and' Fig. 12;, it will be observed that again,; one of the cathodes .16 :or 11 may be ex:- cited: alone shy; :source- .12 I Q to produce the 4 desired result, and itzismot necessary'thatjboth cathodes be excited;

In Fig. 6 there is illustrated: diagrammatically still; another embodiment of: the invention wherein multieelement 1 tubes :are employedas modulating ,unita; Asp-before, 22 again denotes :a si-gnalr-sourceeandflIva local source capable of control by means of; heterodyne" oscillator V 38. The :signal from; source. -22 is applied :in pushpull,.(l80.s out-oft; phase) to :grids 93 and 94 through ,radio .-frequency, transformer 95.1 The signal ftomrsourcei t is -applied to another set of;g11ids; 96;..and; 91:! in phase-through radio ire-V quency transformer 98.-- With-respect-to 031th? odes 99-,-and:l0 l;? ridsfiigand 94; are normally biased essentially -top negative cut-off. by means of potentialasource lfll, andv grids 1 96 and:- 91 are maintained positive by means-of potential source I02 Plates-I03 and. I04 =are maintained-positivewithrespect ,to cathodes'99 and-I00 by means of potentialsource -l Ultacting through equalload impedances=-. lllacand l 01 and then. radio vfree quency-chokes I08, andl09 respectively. Plates I0 3sand l04-are also .bypassedto cathodes 99 'and 1.00; byradiovfrequency bypass; condensers H0 and: l I I; Output'is taken between terminals H2 and; Hithroughcoupling condensers. H4 and I I5 respectively; Under conditions of no excitation from source 21; equal and opposite voltages will beproduced across'loads l06-and I01, correspondinggtothe amplitude modulation envelopes recovered from the 'positiveexcursions of grids-93 and 94 respectively under excitation from source: 22. Therefore; essentially no modulation products will ,be-produced between points H2 and H3 except those which result from non symmetrical excursions of-grids 93and 94; If now, grids 96-and BTareexcited by source 2| at a frequency slightly difierent from that of source 22, the heterodyne-orbeat frequency will appear as-.-a;.:modulation"product between points 2 and H3: And. in-a:manner-analogous to that discussed-abovefor Figs; 1, 4, and 5, amplitude modulatiompeaks-on "the signal from source 22, ifj they exceed in their efiect onthe 'plate current throughloads lllfiiandsl 01', the excursions of grids 96 and'01 011*.SllCh plate current, the resulting plate-currents .will' besin'phase and since added in opposition; will balance out. Thus, under the conditions namedythe circuitofFig. 6 will automatically reject noise peaks above an "arbitrary level; while-:passingthe heterodyneorbeat note requiredrfor radio telegraph signal reception,

Alternatively; by; synchronizing source 2 l" with source 22 and exciting: grids'fifi and'lll 'inphase (0"? phase-difierence) with: either'grid- 93-or 94; through action analogous to that illustratedin Figs. 3 and 3B,-.all;amplitude modulation existing 011113118 signal' iromzsource 22, which does vnot exceed ineffective amplitude an arbitrary level determined'fb y'the excitation from source 2i, will appear as :a; potentialv difference between points I l2;-and'*.l l3; whereas, all? amplitude modulation on the signaliromsZ? which exceeds this level will cancelt'out between-points H2 and"ll3.' Under these latterr'conditions;then; the'circuitiof Fig. 6 iszrsuitable forzthereception of amplitude'modulated signals from; source- 22, v and provides can'- cellation of noise peaks or pulseszwhioharedn excess of any; desired level.-

Again; his to be noted-.that asin thepreccding embodiments; grids :93 1 and 94 may be excited in phase flGmfSOlllCQZZ, anduponexciting' grids 96 and:9'|-180 out. ofzph-asewith each other from source #2 I; similar results will be obtained to those indicated ffopFig: 6 circuit. However, the effect ofsexciting; grids:.-93 and; 94 in. phase is to derive the opposing modulation" envelopes appearing across-=loads-l 06 and. lll'lifrom thesame positive excursions of grids 93 'and'd liandtherebyto renderrthe' nois'e cancellation: independent of' the symmetry'iof signal modulation :from source 22..

It istfurther .tobe'observed that in 6', ifithe excitation from'source '2 l is applied :to eithergrid 96wor '91 singly;rath'er than to both, as for examand 22 to form a modulation product in its output may be used, as for example, any of the various mineral detectors, well known in the art.

In general mathematical form, if the signal from source 22 is represented by:

sin 21rNt Where c is the amplitude of this signal N is the frequency of this signal 1. is time parameter and if the excitation from source 2| is represented by:

1) sin 21rnt where b is the amplitude of excitation n is the frequency of excitation t is time parameter then the necessary and sufiicient conditions of the invention will be met if, first, the respective currents Ix and Iy through the two load impedances (e. g. I2, I3; 63, 64; 84, 85; I06, I01) of a balanced modulator system in a radio receiver can be represented (neglecting harmonic and high order terms) by functional expressions o-fthe type:

Ie=F(21rNt, 21m) c sin 2 m sin 21m: I =F(2irNt, 21m) 0 sin 211w sin 21rnt when b is greater than c and Iz=f(21rNt, 21rnt) -b sin 21rNt sin 21rnt Iy=f(21rNt, 21rnt) b sin 21rN t sin 21 m? when 0 is greater than band if, second, the output voltage E0 be derived from these currents in opposition so that:

when I) is greater than c, and where k is a proportionality constant and From these equations, it is seen that when the amplitude c of the incoming signal from source 22 is less than the amplitude b of excitation from source 2|, a beat frequency of amplitude clc will be present in the output; it is also apparent that amplitude 27, the output voltage is limited by the magnitude of b and. is independent of c. This is the condition for reception of radio telegraph signals. It is further apparent from the above if a peak of noise causes amplitude c to exceedamplitude c is fluctuating because of the ampliever, it is noted that a mathematical statement frequencies may be found in any standard trigapplied to said balanced modulator means to a 10 equations that at the beginning of each train of incoming waves i=0, and if the frequency N+n is filtered out, as for example by condensers M and I5 in Fig. 1, then the expressions for E0 become:

Eo=c7c c0s0 and 'Eo=kb cos0 respectively. If then, in addition, the initial phase difference 0 be made equal to the expressions for E0 vanish. That is, if an initial phase difference of 90 be present between the signals from source 2! and source 22, the output is substantially zero regardless of the values of amplitudes b and 0, until such time as the expression 21](N -11) t has reached a value substantially greater than zero.

For the reception of radio telephone signals, frequencies N and it become identical; N --n=0; and the frequency N+n is filtered out by radio frequency by-pass condensers (for example, by condensers I4 and 15 in Fig. 1). Therefore when the initial phase difierence 0 is equal to 0 or E0=Ck when b is greater than c, and now since tude modulation impressed on it in radio telephone transmission, it is apparent that this same modulation will appear in E0 and the signal will be detected or received. As soon, however, as the amplitude c of source 22 exceeds the amplitude b of excitation from source 2|, then Eo=bk; that is, the output voltage is limited by the amplitude b of source 2| and the modulation of amplitude c disappears above this level. It is also noted that under the conditions where N =n, if the initial phase difierence 0 is equal to 90, E0 becomes zero for all values of amplitudes b and c; i. e., all modulation is cancelled out. For values of 0 intermediate between 0 and 90, the modulation of 0 will be recovered, but will be more reduced in amplitude as 0 approaches 90.

The mathematical steps in the above description are familiar to those skilled in the art; howof the fundamental relations involved in amplitude modulation may be found in Frequency Modulation, pages 1 thru 9, by August Hund; and further, the derivation of the sum and difference frequencies from a product of the two parent onometry book, as for example on page 57 of Elements of Plane and Spherical Trigonometry by Dr. D. A. Rothrock.

While only certain specific embodiments of the invention have been'illustrated and described to convey the general concept of the invention, it is to be understood that the same is readily capable of various other embodiments within its spirit and scope as defined in the appended claims. It is further to be understood that although intended primarily for radio Wave reception, the present invention is equally applicable to the reception of any electrical wave disturbances, as, for example, any of the various carrier systems now in communications use.

I claim:

1. In an electrical wave receiving system, means for receiving signal and noise waves, means for generating local electrical waves, balanced modulator means interconnected with and responsive to excitation from both of said first-named means to produce combinational waves, means constantly limiting the amplitude of the local waves value lower than that of reception-disruptive 11* components of said signal and noise Waves, thereby constantly to limit the amplitude of said combinational waves to a value proportional to the value of the local waves applied to the modulator means, and selective means acting on said combinational waves to segregate the desired ones thereof.

2. An electrical wave receiving system as set forth in claim 1 in which the balanced modulator means comprises two or more balanced modulator elements for the production of said combinational Waves.

3. An electrical wave receiving system as set forth in claim 1 in which the balanced modulator means comprises balanced rectifier-modulator elements for theproductionof said combinational waves.

4. An electrical wave receiving system as set forth in claim 1 in which the balanced modulator means comprises balanced diode electronic rectifier elements for the production of said combinational' waves.

5. An electrical wave receiving system as set forth in claim 1 in which the balanced modulator means comprises balanced multi-element electronic tubes as modulator elements for the production of said combinational waves.

6. An electrical wave receiving system as set forth in claim 1 in which the balanced modulator means comprises at least two balanced modulator elements; and in which the received signal and noise waves are applied to said modulator elements in the same phase for the production of said combinational waves.

7. Anelectrical wave receiving system as set forth in claim 1 in which the balanced modulator means comprises at least two balanced modulator elements; and in which excitation from the, local electrical wave source acts on only one of said modulator elements for the production of said combinational waves. I

8. An electrical wave receiving system as set forth in claim 1 in which the balanced modulator means comprises at least two balanced modulator elements; and in which the local electrical wave source acting on said modulator elements is interconnected with the said signal wave receiving means and is actuated only by the presence of said received signal waves.

9. An electrical wave receiving. system as set.

forth in claim 1 in which the balanced modulator means comprises at least two balanced modulator elements; and in which the local electrical wave source acting on said modulator elementsis interconnected with the said signal and noise'wave receiving means to produce an initial phase difference of 90 between said received waves and the excitation from said local source upon the beginning of each new received wave train. 4

10. An electrical wave receiving systemv asset forth in claim 1 in which the local source-of electrical wave energy is operated at the same frequency as one component of said received signal waves to produce the said cornbinationalwaves.

11. In an electrical wave receiving system, means for generating a local electrical wave'substantially of the form bsin 21rnt, where-b is the amplitude of the local wave, n is the frequency" of the local wave, and t is time parameter; means for receiving signal and noise waves, said signal and noise waves being generally representedby one or more expressions of the type csin 211-Nt,

where c is the instantaneous amplitude of said, waves, N is frequency of said-waves, and t istime parameter; balanced modulator means interconl2 nected with and responsive to excitation from both of saidfirst-named means to produce modulation-product waves of the general form 217 sin 21rnt sin Zn'Nf when 0 is greater than D, and of the general form sin 21rnt sin 21rNt when I) is greater than 0; means constantly limiting the amplitude b to a value lower than reception disruptive values of c,

19 thereby substantially to eliminate reception dis- 20 forth in claim 11 in which the balanced modulator means comprises balanced diode electronic rectifier-tubes for the production of said modulationv product waves.

14; An electrical wave receiving system as set 25 forthin claim 11in which the balanced modulator means comprises balanced multi-element electronic tubes for the production of said modulationproduct waves.

15. An electrical wave receiving'system as set forth in claim 11 in which the balanced modulator means comprises at least two balanced modulator elements; and in. which the said received signal and noise waves are applied to said modulator elements in the same phase for the production of said; modulation. product. waves.

16,..An electrical wave receivin system as set forth in claim 1 1 in which the balanced modulator means comprises at least two balanced modulator elements; and in which excitation from the local electrical wave source acts on only one of said modulator elements for the production of said modulation product waves.

17. An electrical wave receiving-'system'asset forth in claim 11 in which the localnelectrical wave source is inter-connectedawith the said signal and noisewave receiving means and is actuated only during presence of said received waves to produce said modulation product waves.

18. Anelectrical wave receiving systemas-set forth in claim 11 inwhich the local electrical wave source is inter-connected with the said signal and-noise wave receiving means, whereby said received" waves actuate the local source only during the presence of said received waves, and

; whereby an initial phasedifierence of'90is produced between" saidreceived waves and the excitation fromsaid local source by the beginning-of each new received electrical wave train.

19. An electricalwave receiving system as set forth in claim 11;in -whic h the frequency 11. of the local source of electrical wave energy is made equal to the frequency N of the said received signal and noise waves for the-production of modulation product waveswhich may be represented -;by expressions of the general type c(1cos 41rNt) when cis-less than D; and b(-1cos 41rNt) when c is greaterthan b.-

20. In a radio signal receiving system,- means l; ceiving. radio signal and noise waves means I 707 fflrzgfinerating local radio waves; balanced modulater means. interconnected with and, responsive to excitation from both of said first, named means to; produce combinational waves; means constantly limiting. the amplitude of the localwaves. 7551. applied .to, said.- =balanced, modulator. means to a.

tuned receiving circuit and the input circuit of each of said modulator elements, a source of locally generated electrical waves, means for applying said locally generated waves to each of said modulator elements, means constantly limiting the amplitude of said local waves applied to said balanced modulator means to a value lower than that of reception-disruptive components of said signal and noise waves, and an output system differentially connected to the output circuits of said modulator elements.

IVAN W. CONRAD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,038,285 Harris Apr, 21, 1936 1,977,536 Tubbs Oct. 16, 1934 1,918,433 Smythe July 18, 1933 1,988,621 Hansell Jan. 22, 1935 Re. 17,360 Gage July 2, 1929 2,256,199 Hansell Sept. 16, 1941 1,959,275 McCaa May 15, 1934 1,521,380 McCaa Dec. 30, 1924 1,344,275 McCaa June 22, 1920 1,885,009 Day Oct. 25, 1932 1,343,308 Carson June 15, 1920 1,244,697 Carson Oct. 30, 1917 2,040,221 Tuhbs May 12, 1936 2,103,878 Thompson Dec. 28, 1937 2,240,500 Gabrilovitch May 6, 1941 Certificate of Correction I Patent No. 2,424,925. July 29, 1947..

IVAN W. CONRAD It is hereb certified that error appears in the above numbered patent requiring correction as f0 lows: In the drawing, Figure 4 should appear as shown below instead of as shown in the patentand that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 18th day of November, A. D. 1947.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

